diff --git a/CHANGELOG.md b/CHANGELOG.md
index ae6725702..e084de447 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,4 +1,5 @@
 # Changelog
+- Provide option to fix foreign investments outside Germany to optimized capacities of reference scenario
 - Longer lifetime (40 years) is only applied to existing gas CHPs, not new ones. Added a new config entry `existing_capacities:fill_value_gas_chp_lifetime`
 - Bugfix: gas CHPs are extendable again
 - Simplified scenarion definition and made `Mix` the default scenario
diff --git a/Snakefile b/Snakefile
index a80748ad0..6e52636ef 100644
--- a/Snakefile
+++ b/Snakefile
@@ -519,6 +519,17 @@ rule modify_district_heat_share:
         "scripts/pypsa-de/modify_district_heat_share.py"
 
 
+def get_reference_network(w):
+    ref_scenario = config_provider("fix_foreign_investments", "reference_scenario")(w)
+    if (
+        config_provider("fix_foreign_investments", "enable")(w)
+        and w.run != ref_scenario
+    ):
+        return f"results/{config_provider("run", "prefix")(w)}/{ref_scenario}/networks/base_s_{w.clusters}_{w.opts}_{w.sector_opts}_{w.planning_horizons}.nc"
+    else:
+        return []
+
+
 rule modify_prenetwork:
     params:
         efuel_export_ban=config_provider("solving", "constraints", "efuel_export_ban"),
@@ -553,6 +564,10 @@ rule modify_prenetwork:
         shipping_methanol_share=config_provider("sector", "shipping_methanol_share"),
         mwh_meoh_per_tco2=config_provider("sector", "MWh_MeOH_per_tCO2"),
         scale_capacity=config_provider("scale_capacity"),
+        fix_foreign_investments=lambda w: config_provider("fix_foreign_investments")(w),
+        enable_fix_foreign_investments=lambda w: config_provider(
+            "fix_foreign_investments", "enable"
+        )(w),
     input:
         costs_modifications="ariadne-data/costs_{planning_horizons}-modifications.csv",
         network=resources(
@@ -581,6 +596,7 @@ rule modify_prenetwork:
         regions_onshore=resources("regions_onshore_base_s_{clusters}.geojson"),
         regions_offshore=resources("regions_offshore_base_s_{clusters}.geojson"),
         offshore_connection_points="ariadne-data/offshore_connection_points.csv",
+        reference_network=get_reference_network,
     output:
         network=resources(
             "networks/base_s_{clusters}_{opts}_{sector_opts}_{planning_horizons}_final.nc"
diff --git a/config/config.de.yaml b/config/config.de.yaml
index 22bfcc13d..367a19193 100644
--- a/config/config.de.yaml
+++ b/config/config.de.yaml
@@ -4,7 +4,7 @@
 
 # docs in https://pypsa-eur.readthedocs.io/en/latest/configuration.html#run
 run:
-  prefix: 20253006_fix_chp_lifetime
+  prefix: 20250714_fix_neighbours
   name:
   # - ExPol
   - KN2045_Mix
@@ -77,6 +77,13 @@ existing_capacities:
 # Germany plus 12 "Stromnachbarn"
 countries: ['AT', 'BE', 'CH', 'CZ', 'DE', 'DK', 'FR', 'GB', 'LU', 'NL', 'NO', 'PL', 'SE', 'ES', 'IT']
 
+fix_foreign_investments:
+  enable: false
+  reference_scenario: KN2045_Mix
+  nom_min: true
+  nom_max: true
+  slack: 1.e-5
+
 # docs in https://pypsa-eur.readthedocs.io/en/latest/configuration.html#snapshots
 snapshots:
   start: "2019-01-01"
diff --git a/rules/solve_myopic.smk b/rules/solve_myopic.smk
index 1df6afeec..10119d35a 100644
--- a/rules/solve_myopic.smk
+++ b/rules/solve_myopic.smk
@@ -143,6 +143,9 @@ rule solve_sector_network_myopic:
         ),
         custom_extra_functionality=input_custom_extra_functionality,
         energy_year=config_provider("energy", "energy_totals_year"),
+fix_foreign_investments=config_provider(
+            "fix_foreign_investments"
+        )
     input:
         network=resources(
             "networks/base_s_{clusters}_{opts}_{sector_opts}_{planning_horizons}_final.nc"
diff --git a/scripts/pypsa-de/modify_prenetwork.py b/scripts/pypsa-de/modify_prenetwork.py
index 9ead691e4..7110dff9e 100644
--- a/scripts/pypsa-de/modify_prenetwork.py
+++ b/scripts/pypsa-de/modify_prenetwork.py
@@ -6,7 +6,13 @@
 import pypsa
 from shapely.geometry import Point
 
-from scripts._helpers import configure_logging, mock_snakemake, sanitize_custom_columns
+from scripts._helpers import (
+    configure_logging,
+    mock_snakemake,
+    sanitize_custom_columns,
+    set_scenario_config,
+    update_config_from_wildcards,
+)
 from scripts.add_electricity import load_costs
 from scripts.prepare_sector_network import lossy_bidirectional_links
 
@@ -1265,6 +1271,212 @@ def scale_capacity(n, scaling):
                 ]
 
 
+def _get_component_pair(n, n_ref, component_type):
+    """
+    Get component dataframes from both networks.
+
+    Parameters
+    ----------
+    n : pypsa.Network
+        Network object to modify.
+    n_ref : pypsa.Network
+        Reference network object.
+    component_type : str
+        Component type name.
+
+    Returns
+    -------
+    tuple
+        (component, baseline_component) dataframes.
+    """
+    if component_type == "StorageUnit":
+        component = getattr(n, "storage_units")
+        baseline_component = getattr(n_ref, "storage_units")
+    else:
+        component = getattr(n, component_type.lower() + "s")
+        baseline_component = getattr(n_ref, component_type.lower() + "s")
+    return component, baseline_component
+
+
+def _identify_non_german_extendable(component, component_type):
+    """
+    Identify non-German extendable components.
+
+    Parameters
+    ----------
+    component : pd.DataFrame
+        Component dataframe.
+    component_type : str
+        Component type name.
+
+    Returns
+    -------
+    pd.Series
+        Boolean series indicating components to fix.
+    """
+    # Identify non-German nodes
+    if component_type in ["Line", "Link"]:
+        # For lines and links, check both buses
+        non_german = component.filter(regex="bus[012]").apply(
+            lambda x: (~x.str.startswith("DE").any()) & (not x.name.startswith("EU")),
+            axis=1,
+        )
+    else:
+        # For other components, check if bus is not in Germany
+        non_german = ~component.bus.str.startswith(("DE", "EU"))
+
+    # Only fix extendable components
+    extendable = (
+        component.e_nom_extendable
+        if component_type == "Store"
+        else (
+            component.s_nom_extendable
+            if component_type == "Line"
+            else component.p_nom_extendable
+        )
+    )
+    return non_german & extendable
+
+
+def _apply_capacity_limits(
+    n, component_type, indices, baseline_component, slack, nom_min, nom_max
+):
+    """
+    Apply capacity limits to components.
+
+    Parameters
+    ----------
+    n : pypsa.Network
+        Network object to modify.
+    component_type : str
+        Component type name.
+    indices : pd.Index
+        Indices of components to modify.
+    baseline_component : pd.DataFrame
+        Reference component dataframe.
+    slack : float
+        Slack factor to apply to capacity limits.
+    nom_min : bool
+        Whether to set minimum capacity limit.
+    nom_max : bool
+        Whether to set maximum capacity limit.
+    """
+    # Determine capacity attributes based on component type
+    if component_type == "Store":
+        nom_attr, nom_opt_attr, nom_min_attr, nom_max_attr, extendable_attr = (
+            "e_nom",
+            "e_nom_opt",
+            "e_nom_min",
+            "e_nom_max",
+            "e_nom_extendable",
+        )
+        component_df = n.stores
+    elif component_type == "Line":
+        nom_attr, nom_opt_attr, nom_min_attr, nom_max_attr, extendable_attr = (
+            "s_nom",
+            "s_nom_opt",
+            "s_nom_min",
+            "s_nom_max",
+            "s_nom_extendable",
+        )
+        component_df = n.lines
+    else:
+        nom_attr, nom_opt_attr, nom_min_attr, nom_max_attr, extendable_attr = (
+            "p_nom",
+            "p_nom_opt",
+            "p_nom_min",
+            "p_nom_max",
+            "p_nom_extendable",
+        )
+        component_df = n.df(component_type)
+
+    if nom_min and nom_max and slack == 0:
+        # If both min and max are set, use optimized value directly
+        component_df.loc[indices, nom_attr] = baseline_component.loc[
+            indices, nom_opt_attr
+        ]
+        component_df.loc[indices, extendable_attr] = False
+    else:
+        if nom_min:
+            component_df.loc[indices, nom_min_attr] = baseline_component.loc[
+                indices
+            ].apply(
+                lambda row: max(
+                    np.floor(row[nom_opt_attr]) * (1 - slack),
+                    row[nom_min_attr],
+                ),
+                axis=1,
+            )
+        if nom_max:
+            component_df.loc[indices, nom_max_attr] = baseline_component.loc[
+                indices
+            ].apply(
+                lambda row: min(
+                    np.ceil(row[nom_opt_attr]) * (1 + slack),
+                    row[nom_max_attr],
+                ),
+                axis=1,
+            )
+
+
+def fix_foreign_investments(n, n_ref, slack=0, nom_min=True, nom_max=False):
+    """
+    For all extendable components located outside Germany, this function sets their
+    minimum and maximum capacity limits to match the optimized capacity from a
+    reference network. This effectively fixes the investment decisions for these
+    components to their reference values.
+
+    Components are identified as non-German based on their bus location:
+    - For Line and Link: any connected bus is outside Germany
+    - For other components: the primary bus is outside Germany
+
+    Capacities of EU components are not fixed.
+
+    Only components with extendable capacity are modified (those with
+    [p|s|e]_nom_extendable set to True).
+
+    Parameters
+    ----------
+    n : pypsa.Network
+        Network object to modify.
+    n_ref : pypsa.Network
+        Reference network object containing optimized capacity values.
+    slack : float, optional
+        Slack factor to apply to the capacity limits. Default is 0.
+    nom_min : bool, optional
+        Whether to set the minimum capacity limit. Default is True.
+    nom_max : bool, optional
+        Whether to set the maximum capacity limit. Default is False.
+
+    Returns
+    -------
+    None
+        Network is modified in-place with updated capacity limits.
+    """
+    # List of component types that can have investment decisions
+    investment_components = ["Generator", "StorageUnit", "Store", "Link", "Line"]
+
+    # For each component type
+    for component_type in investment_components:
+        component, baseline_component = _get_component_pair(n, n_ref, component_type)
+
+        to_fix = _identify_non_german_extendable(component, component_type)
+
+        if not any(to_fix):
+            continue
+
+        indices = component.index[to_fix]
+
+        # Set optimized capacity from reference network as lower and upper
+        # bound rounding values to the nearest integer and inserting slack
+        # to avoid constraint violations
+        _apply_capacity_limits(
+            n, component_type, indices, baseline_component, slack, nom_min, nom_max
+        )
+
+        logger.info(f"Fixed {sum(to_fix)} {component_type} components outside Germany")
+
+
 if __name__ == "__main__":
     if "snakemake" not in globals():
         snakemake = mock_snakemake(
@@ -1274,11 +1486,13 @@ def scale_capacity(n, scaling):
             opts="",
             ll="vopt",
             sector_opts="none",
-            planning_horizons="2025",
-            run="KN2045_Mix",
+            planning_horizons="2045",
+            run="KN2045_Mix_10solarCAPEX_notfixed",
         )
 
     configure_logging(snakemake)
+    set_scenario_config(snakemake)
+    update_config_from_wildcards(snakemake.config, snakemake.wildcards)
     logger.info("Adding Ariadne-specific functionality")
 
     n = pypsa.Network(snakemake.input.network)
@@ -1347,4 +1561,21 @@ def scale_capacity(n, scaling):
 
     sanitize_custom_columns(n)
 
+    if (
+        snakemake.params["fix_foreign_investments"]["enable"]
+        and snakemake.wildcards.run
+        != snakemake.params["fix_foreign_investments"]["reference_scenario"]
+    ):
+        logger.info(
+            "Fixing investments for components outside Germany based on the reference scenario."
+        )
+        n_ref = pypsa.Network(snakemake.input.reference_network)
+        fix_foreign_investments(
+            n,
+            n_ref,
+            snakemake.params["fix_foreign_investments"]["slack"],
+            snakemake.params["fix_foreign_investments"]["nom_min"],
+            snakemake.params["fix_foreign_investments"]["nom_max"],
+        )
+
     n.export_to_netcdf(snakemake.output.network)
diff --git a/scripts/solve_network.py b/scripts/solve_network.py
index fdc950487..418c881a1 100644
--- a/scripts/solve_network.py
+++ b/scripts/solve_network.py
@@ -183,7 +183,12 @@ def add_land_use_constraint(n: pypsa.Network, planning_horizons: str) -> None:
     n.generators["p_nom_max"] = n.generators["p_nom_max"].clip(lower=0)
 
 
-def add_solar_potential_constraints(n: pypsa.Network, config: dict) -> None:
+def add_solar_potential_constraints(
+    n: pypsa.Network,
+    config: dict,
+    fix_foreign_investments: dict,
+    run: str,
+) -> None:
     """
     Add constraint to make sure the sum capacity of all solar technologies (fixed, tracking, ets. ) is below the region potential.
 
@@ -201,14 +206,29 @@ def add_solar_potential_constraints(n: pypsa.Network, config: dict) -> None:
     rename = {} if PYPSA_V1 else {"Generator-ext": "Generator"}
 
     solar_carriers = ["solar", "solar-hsat"]
+    if fix_foreign_investments.get("enable") and run != fix_foreign_investments.get(
+        "reference_scenario"
+    ):
+        buses_to_constrain = n.buses.query("country == 'DE'").index
+    else:
+        buses_to_constrain = n.buses.index
+
     solar = n.generators[
-        n.generators.carrier.isin(solar_carriers) & n.generators.p_nom_extendable
+        n.generators.carrier.isin(solar_carriers)
+        & n.generators.p_nom_extendable
+        & n.generators.bus.isin(buses_to_constrain)
     ].index
 
     solar_today = n.generators[
-        (n.generators.carrier == "solar") & (n.generators.p_nom_extendable)
+        (n.generators.carrier == "solar")
+        & (n.generators.p_nom_extendable)
+        & n.generators.bus.isin(buses_to_constrain)
+    ].index
+
+    solar_hsat = n.generators[
+        (n.generators.carrier == "solar-hsat")
+        & n.generators.bus.isin(buses_to_constrain)
     ].index
-    solar_hsat = n.generators[(n.generators.carrier == "solar-hsat")].index
 
     if solar.empty:
         return
@@ -1200,7 +1220,12 @@ def extra_functionality(
     ) and {"solar-hsat", "solar"}.issubset(
         config["electricity"]["extendable_carriers"]["Generator"]
     ):
-        add_solar_potential_constraints(n, config)
+        add_solar_potential_constraints(
+            n,
+            config,
+            snakemake.params["fix_foreign_investments"],
+            snakemake.wildcards.run,
+        )
 
     if n.config.get("sector", {}).get("tes", False):
         if n.buses.index.str.contains(